1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image. More particularly, the present invention relates to a fixing device for use in electrophotographic image forming apparatus, such as copiers, facsimile machines and laser printers, which fixes toner images formed by a direct or indirect monochrome electrophotographic developing method or a direct or indirect full color electrophotographic developing method. In addition, the present invention also relates to an image forming apparatus using the fixing device.
2. Discussion of the Background
When a toner image formed by an electrophotographic image forming method is fixed, heat fixing methods using a heat roller are typically used. In heat roller fixing methods, a toner image formed on a recording material is contacted and heated with a heat roller, which has a good releasability from the toner, upon application of pressure to the toner image, resulting in fixation of the toner image on the recording material. The heat roller fixing methods have an advantage of having excellent heat efficiency in fusing a toner image because the toner image to be fixed contacts the surface of a heating roller (i.e., a fixing roller) while being pressed by a pressing roller. Therefore, toner images can be rapidly fixed on recording materials.
However, the heat roller fixing methods tends to cause an offset problem in that part of a fused toner image, which contacts the surface of a heating roller under pressure, is adhered and transferred to the surface of the heating roller, and then the part of the toner image is re-transferred to an undesired portion of the sheet itself or the following sheet of the recording material. Whether or not the offset problem occurs largely depends on the fixing speed and the fixing temperature of the fixing device used.
In general, when the fixing speed is high, the temperature of the surface of the heating roller is set to be a relatively high, to apply substantially a constant amount of heat to the toner image to be heated independently of the fixing speed.
When a full color image is formed, a plurality of color toner layers are overlaid on a recording material. If such a full color toner image is fixed with a heating roller at a high speed, the difference in temperature of the uppermost toner layer and the lowermost toner layer is large. In this case, the heating roller tends to be heated to a high temperature to fully fix the lowermost toner layer. Therefore, the uppermost toner layer is heated to a high temperature, and thereby a hot offset problem tends to be caused. If the temperature of the heating roller is decreased to avoid the hot offset problem, another problem in that the lowermost toner layer is not fully fixed, resulting in occurrence of a cold offset phenomenon in that the unfixed toner layer adheres to the heating roller.
In attempting to solve the problems, a method in which when the fixing speed is high, the fixing pressure is increased so that the toner image to be fixed is anchored to a recording material is used. In this case, the temperature of the heat roller can be decreased to some extent, and thereby the hot offset problem of the uppermost toner layer can be avoided. However, the shearing force applied to the toner image to be fixed is increased, and thereby another problem in that the recording sheet is wound around the heating roller, resulting in jamming of the recording sheet. Alternatively, even when such a jamming problem does not occur, a problem in that the resultant image has a white scratch image because the toner image is scratched with a separation pick which is configured to separate the recording material from the heating roller tends to occur. In addition, when the fixing pressure is high, problems in that fine line images are widened, and/or toner particles in the toner image are scattered tend to occur, resulting in deterioration of image qualities.
With respect to toner, a toner having a relatively low melt viscosity at the fixing temperature is typically used when the fixing speed is relatively high. In this case, the temperature and pressure of the heating roller are decreased to avoid the hot offset problem and the jamming problem (i.e., the recording paper winding problem). On the other hand, recently a need exists for a toner which can be used in a wide fixing speed range without causing the offset problems and the jamming problem. When such a toner is used at a low fixing speed, the hot offset problem tends to occur. Namely, there is no toner which can be used in a wide fixing speed range.
On the other hand, recently a need exists for high quality images having good fine line reproducibility. In attempting to fulfill this need, the particle diameter of toner is decreased more and more. When such a toner having a small particle diameter is used, the fixing properties of half tone images deteriorate particularly when the fixing speed is relatively high. This is because the amount of toner particles in a half tone image is relatively small and the amounts of heat and pressure applied to the half tone image are small particularly when the half tone image is formed on a recessed portion of a recording material. In addition, the shearing force applied to one toner particle in a half tone image formed on a projected portion of a recording material is relatively high compared to that applied to a toner particles in a solid image, because the toner layer in the half tone image is thin. Therefore, the offset problem easily occurs at the projected portion. Thus, the resultant half tone image tends to have poor image qualities.
In attempting to impart a good combination of fixability and offset resistance to toner, the binder resins therefor have been actively investigated. For example, published unexamined Japanese Patent Application No. 05-107803 (hereinafter referred to as JP-A) discloses a technique in that a resin having a molecular weight distribution such that at least one peak is present in each of the ranges of from 103 to 7×104 and from 105 to 2×106 is used as a binder resin.
JP-As 05-289399 and 05-313413 have disclosed a technique in that a vinyl copolymer having a specific molecular weight is used as a binder resin, and a release agent such as polyethylene is used in combination therewith, to impart a good combination of fixing property and offset resistance to the resultant toner.
JP-A 05-297630 discloses a technique in that a combination of a resin having a low melt viscosity and a resin having a high melt viscosity is used as a binder resin to improve the low temperature fixability and hot offset resistance of the toner.
In addition, JP-As 05-053372 and 06-118702 have disclosed a technique in that a resin having a relatively wide molecular weight distribution is used as a binder resin to impart a well-balanced combination of preservability, fixability and hot offset resistance to the resultant toner.
Conventional electrophotographic image forming apparatus typically includes a fixing device in which a recording material bearing a toner image thereon is fed through a nip between a heating roller having a heat source therein and a pressing roller which presses the recording material toward the heating roller while feeding the recording material, to fix the toner image on the recording material.
As mentioned above, such a fixing device tends to cause the offset problem in that the toner image on the recording material is adhered to the heating roller, and the toner image is re-transferred to an undesired portion of the sheet or the following sheet of the recording material. When such offset phenomenon is caused, the toner image adhered to the heating roller is also transferred to the pressing roller contacting the heating roller, and thereby a problem in that a toner image is re-transferred to the backside of the recording sheet or the following recording sheet is also caused. In attempting to avoid such an offset problem, a technique in that the surface of the heating roller is coated with a fluorine-containing compound is proposed. However, even though such a heating roller is used, it is impossible to perfectly prevent occurrence of the offset problem when the environmental conditions and the recording materials are changed.
In order to prevent heating rollers and pressing rollers from being contaminated with transferred toner particles, a technique in that a cleaning device is provided so as to contact the heating rollers and the pressing rollers is proposed. For example, a cleaning device having a metal roller is used. The metal roller is contacted with the heating rollers and pressing rollers to catch the toner particles thereon utilizing difference in toner releasability between the metal roller and the heating rollers and pressing rollers, which are treated to a toner releasing treatment.
Recent image forming apparatus typically adopt a fixing method in which a power is not applied to a heat source of the fixing device thereof in a waiting time to eliminate waste electric consumption, and a power is timely applied thereto when an image forming operation is ordered. Therefore, it is necessary to rapidly increase the temperature of the heating roller of the fixing device to the fixing temperature, i.e., it is necessary to improve the heat response of the heating roller. Therefore, a roller having a thickness not greater than 1 mm, which can be heated to the fixing temperature within a time of about 10 seconds, is typically used as the heating roller.
In such image forming apparatus using a heating roller with a small heat capacity, the temperature of the heating roller tends to be influenced by heat transfer from the heating roller to recording sheets and the members contacting the heating roller and other factors such as direction of flow of air surrounding the heating roller. Therefore, the image forming apparatus has a problem in that the temperature of the heating roller varies in the width direction thereof. However, this problem is not yet solved, namely it is difficult to uniformly heat the entire of such a heating roller to a predetermined fixing temperature because of the structural limitation and cost limitation.
When the temperature of a heating roller becomes uneven in the width direction thereof, good fixing properties cannot be stably obtained and the offset problem tends to be caused. In addition, the life of the heating roller is shortened due to heat deterioration of the heating roller. In particularly, when the polymerized toner disclosed in, for example, JP-A 2000-292981 is used, a problem in that toner blocks adhered to and accumulated on a cleaning member are re-melted and transferred to a recording sheet occurs. When a toner prepared by a pulverization method is used, such a problem hardly occurs. This is because in the case of the pulverization toners, toner particles having a high storage modulus are typically adhered to such a cleaning member whereas toner particles with a low storage modulus are adhered to the cleaning member in the case of polymerization toners.
This problem is typically caused when recording sheets with a narrow width are used. The reason therefor is considered as follows. The temperature of only the central portion of the heating roller contacting the narrow recording sheets is decreased, and the heat source for the heating roller heats entire the heating roller because the temperature fall of the central portion of the heating roller is detected by a temperature sensor provided on the central portion of the heating roller. Therefore, the temperature of both the end portions of the heating rollers is excessively increased, and thereby the toner blocks present on both end portions of the cleaning member are fused and re-transferred to the pressing roller and heating roller.
In attempting to solve this problem, JP-A 09-325550 discloses a technique in that air blows both end portions of a heating roller to prevent the temperature of the portions from being excessively raised. In addition, JP-A 2002-123119 discloses a technique in that air holes are provided along the cleaning roller so that air in the fixing device is circulated with rotation of the cleaning roller, thereby preventing the temperature of the cleaning roller from being excessively raised.
Because of these reasons, a need exists for a fixing device which includes a cleaning device and which does not cause the toner re-transferring problem in that toner particles adhered to the cleaning member is not re-transferred to the fixing member.